Just bought a (second hand) 105 triple chainset as my existing compact double wasn’t for me. However, and I should have checked this before, the Sora STI will only index for a double, despite having space on the gear indicator for three chainrings. There’s nothing I can do about this, is there?

is it all cabled up still – sure it’s not the stops on the mech itself preventing the shifter from operating. or a double specific mech? If you undo the cable from the mech can you get three indices then?